Switching power supplies are popular for high power applications because of the high efficiency and small area and volume required. Buck converters in particular are well suited to providing the high current at low voltages needed by high performance digital integrated circuits such as microprocessors, graphics processors, and network processors. For example, a buck converter is often used to step down a DC voltage (typically referred to as the input voltage) to a lower DC voltage (typically referred to as the output voltage). Since the power stage is fully switched (i.e., the power MOSFET is fully off or on), there is very little loss in the power stage and the converter power efficiency is very high.
In addition to monitoring the output voltage, controllers may also monitor load current. One way to sense the load current is through the output inductor and/or transistor. The current through the output inductor and transistor is representative of the instantaneous load current, which may be used to implement adaptive voltage positioning (AVP), optimize the control mechanism (current mode control), limit the maximum output current, or monitor the operating condition of the regulator. The inductor and transistor current also represents the current through the power devices, the transistors, and the inductor, all of which have maximum current limitations which must be adhered to. Further, in multiphase buck converters, sensing current in each phase allows the controller to balance the output current between all the phases.
RDSon current sense is a popular technique for monitoring the load current because it is a lossless current sense method, and is relatively easy to implement with just a few additional components. Further, the DC level of the signal is guaranteed to be near ground regardless of the regulated output voltage. For example, referring to FIGS. 1A-B, a widely used lossless RDSon current sense model senses current according to the resistance (RDSon) of a transistor in series with a buck inductor, such as a power MOSFET in the output stage of a converter for periodically connecting the load to the input voltage. When the MOSFET is activated, the MOSFET exhibits a resistance RFET, which may be modeled as a separate resistance (FIG. 1B). Assuming the RDSon of the MOSFET is known, the output current may be determined according to the voltage across the MOSFET divided by the resistance of the resistor RFET.